The goal of the rulemaking is to ensure that individuals with disabilities can travel with animals they rely on for assistance while also preventing fraud by travelers bringing along pets under the guise of service animals.

Sara Nelson, president of the Association of Flight Attendants-CWA, which represents 50,000 flight attendants at 20 airlines, urged the department to clarify the regulations for emotional-support animals quickly to avoid interfering with legitimate service animals for veterans and others with disabilities.

"In recent years, there has been an exponential increase of people claiming the need for emotional support animals in the cabin,” Nelson said. “Flight attendants and passengers have been bitten, attacked and inconvenienced by animals who are not properly trained to be in a confined public environment.”

The Americans with Disabilities Act recognized dogs and miniature horses as trained service animals. The Air Carrier Access Act then said service animals could fly in the cabin with passengers, while also opening the door to broader range of emotional-support animals that could travel uncaged to assist passengers with mental-health issues.

Passengers could also bring pets such as dogs or cats in containers that fit in the seating area for a fee. Airlines set their own policies for the fee and the number of animals on a given flight.

But as the variety of animals on flights multiplied, so did complaints. The department received 2,443 complaints from travelers with service animals on U.S. and foreign airlines in 2016 and another 2,499 last year.

In recent years, the variety of emotional-support animals exploded to include monkeys, pigs and ducks as emotional-support animals, although airlines didn't have to accept reptiles, ferrets or rodents.

"The use of unusual species as service animals has also added confusion," said the 41-page filing requesting comment from James Owens, the department's deputy general counsel.

Meanwhile, the Psychiatric Service Dog Society asked the department in 2009 to recognize a new classification for psychiatric service animals distinct from comfort animals. The group asked the department to stop allowing airlines to require documentation and 48 hours' notice for their animals.

Airlines decided to revise their own rules after a Transportation Department panel was unable to reach a regulatory compromise in 2016.

"Recently, a few airlines have begun requiring service animal users to provide information about their animal's health and behavior as a condition of travel," the department said.

In January, Delta said its changes came as the airline carried about 250,000 animals last year that were increasingly misbehaving by wandering the cabin, defecating or even biting passengers. A comfort dog bit a passenger in the face while a flight boarded last June.

United's change in February came after a woman tried to bring a peacock with her on a flight. But United began reviewing its policy in 2017 after noticing a jump in comfort animals on flights to 76,000 from 43,000 the year before and “a significant increase in onboard incidents.”

Starting July 1, American will require passengers to notify the carrier about a comfort animal 48 hours before a flight and then sign a waiver stating the need for the animal.

In order for an animal to qualify, the passenger must provide a letter from a mental-health professional describing the mental or emotional disability that shows the need for the animal and proof of the professional’s licensing.